Further Reading

Walnut f256

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Code: t10
Latin name: Juglans californica
Source material: Pollen
Family: Juglandaceae
Common names: California Black Walnut, California walnut, Jupiter’s Nuts, Carya persica (Greek), Carya basilike (Greek)

Allergen Exposure

Geographical distribution
The Juglandaceae family contains 2 important genera: Hickory/Pecan (Carya) and Walnut (Juglans).

The Walnut tree grows to a height of 12 to 20m, with a large, spreading, rounded top, and thick, massive stem. Some Walnut trees are 300 years old. Black Walnut is the tallest of the Walnuts, with the potential to reach 100 feet. The Walnut tree has compound leaves and consisting of small yellowish-green leaflets.

Walnut is the common name given to twenty species of deciduous trees in the genus Juglans, of which six species are native to the United States. Walnut is native to California, but about 15 related species occur in North and South America as well as in central and southern parts of Europe and Asia.

The Black Walnut is native to the eastern United States and is important for its timber and used in fine furniture rather than for its nut. The Common or English Walnut is native to areas stretching from the Balkans to China, but now widely grown in many other temperate areas, for nut production.

The flowers of separate sexes are borne upon the same tree and appear in early spring before the leaves. The Walnut tree flowers and produces pollen after 20 to 30 years of growth, in late spring to early summer. The pollen of all these trees is large and does not travel far. However, in areas where the trees are cultivated commercially, heavy exposure to the pollen can occur. Walnut pollen is generally considered to be moderately allergenic. The western species of Walnut (in California) is thought to be a more important cause of allergic sensitization than the Black Walnut. The Walnut fruit is a nut, borne singly or in pairs.

Walnut pollens are often the cause of inhalant allergies, and the nuts may cause food allergy.

It occurs in woods and on mountain slopes and may be cultivated in orchards for nut production.

Black Walnut heartwood is heavy, hard, strong, and durable, with a chocolate-brown color prized by furniture manufacturers and many other industries. The Black Walnut is much oilier and richer tasting than the English Walnut found in grocery stores.

Some plants planted near or under the Black Walnut tree tend to yellow, wilt, and die. This occurs because the Walnut tree produces a non-toxic, colorless chemical called hydrojuglone. Hydrojuglone is found in leaves, stems, fruit hulls, inner bark and roots. Several related trees such as English Walnut, hickories and pecan also produce juglone, but in smaller amounts compared to black Walnut.

No allergens from this plant have yet been characterised.

Potential cross-reactivity

Cross-reactivity could be expected between species of the genus Juglans, and on a moderate level to the genus Carya, e.g., Pecan tree (1).

The Walnut tree nut contains a lipid transfer protein allergen (LTP) (2-3). Whether a similar LTP allergen is present in Walnut tree pollen has not been determined yet. Cross-reactivity due to LTP allergens appears to be relevant only in foods, which are ingested, and not in pollens, which are inhaled.

Clinical Experience

IgE-mediated reactions
Anecdotal evidence suggests that asthma, allergic rhinitis and allergic conjunctivitis are common following exposure to pollen from Walnut tree; however, few specific studies have been reported to date (4-5).

A survey of the atmosphere of Bitlis, Turkey, reported that pollen from Juglans spp. was found (6).

Other reactions
Occupational allergic contact dermatitis to Walnut tree wood dust has been observed (7).
Asthma and rhinitis due to the related Central American walnut (Juglans olanchana) dust in a 48-year-old man. Intradermal tests were negative but inhalation with the extract resulted in immediate bronchospasm. Serum IgE antibodies werenot detected (8).
Compiled by Dr Harris Steinman, harris@zingsolutions.com


  1. Yman L. Botanical relations and immuno-logical cross-reactions in pollen allergy. 2nd ed. Pharmacia Diagnostics AB. Uppsala. Sweden. 1982: ISBN 91-970475-09
  2. Asero R, Mistrello G, Roncarolo D, Amato S, van Ree R. A case of allergy to beer showing cross-reactivity between lipid transfer proteins. Ann Allergy Asthma Immunol 2001;87(1):65-7
  3. Asero R, Mistrello G, Roncarolo D, de Vries SC, Gautier MF, Ciurana CL, Verbeek E, Mohammadi T, Knul-Brettlova V, Akkerdaas JH, Bulder I, Aalberse RC, van Ree R. Lipid transfer protein: a pan-allergen in plant-derived foods that is highly resistant to pepsin digestion. Int Arch Allergy Immunol 2000;122(1):20-32
  4. Shafiee A. Atmospheric pollen counts in Tehran, Iran, 1974. Pahlavi Med J 1976;7(3):344-51
  5. Lewis WH, Imber WE. Allergy epidemiology in the St. Louis, Missouri, area. III. Trees. Ann Allergy 1975;35(2):113-9
  6. Celenk S, Bicakci A. Aerobiological investigation in Bitlis, Turkey. Ann Agric Environ Med 2005;12(1):87-93
  7. Estlander T, Jolanki R, Alanko K, Kanerva L. Occupational allergic contact dermatitis caused by wood dusts. Contact Dermatitis 2001;44(4):213-7
  8. Bush RK, Clayton D. Asthma due to Central American walnut (Juglans olanchana) dust. Clin Allergy 1983;13(4):389-94


As in all diagnostic testing, the diagnosis is made by the physican based on both test results and the patient history.